The present invention relates to a chemical vapor deposition method of forming a thin film on a substrate and an apparatus therefor.
Chemical Vapor Deposition (to be referred to as "CVD" hereinafter) is a method of depositing a thin film on a substrate using a chemical reaction and is one of known thin film formation methods such as vacuum evaporation or sputtering. Especially in a manufacturing process of a semiconductor integrated circuit device, CVD is widely used to form a silicon dioxide film or a polycrystalline silicon thin film. CVD is classified into various types in accordance with a heating method, as gas pressure, or a chemical reaction. For example, conventional known CVD methods are: a cold wall type in which only a deposition substrate is heated; a hot wall type in which an entire reaction chamber is heated; atmospheric CVD in which reaction occurs in an atmospheric pressure; low-pressure CVD in which reaction occurs at a low pressure; and plasma CVD and photo CVD in addition to conventional CVD which uses heat to cause chemical reaction.
The characteristics of CVD as a thin film formation means are such that a thin film with good step coverage can be formed on a substrate having projections, that a composition ratio of a thin film can be arbitrarily controlled, and that a thin film can be formed without contamination or damaging a substrate.
However, CVD has drawbacks as compared with vacuum evaporation or sputtering. First, a temperature of a thin film deposition substrate is limited by a reaction temperature of a gas, so that a substrate temperature cannot be arbitrarily changed For this reason, various problems are posed in CVD, especially in normal CVD using heat.
In CVD, a temperature at which deposition progresses at a sufficiently high rate by chemical reaction of a source gas is not always a temperature most suitable for crystal growth of a thin film. A reaction temperature of a source gas in CVD is normally much higher than a substrate temperature in another formation method such as vacuum evaporation or sputtering. When a thin film is deposited on a substrate at a high substrate temperature in a manufacturing process of a semiconductor IC, diffusion or reaction progresses between a deposited film and a substrate or between different materials of a substrate, often resulting in serious degradation in device characteristics.
In thermal CVD, selective growth can be performed utilizing a difference between surface materials of a substrate, i.e., a thin film can be deposited only on a pattern of a specific material of a substrate surface but not on the other material. However, when the substrate temperature is increased, selectivity is decreased by the reason to be described later, so that selective growth cannot be performed well.
In addition, when an alloy film of aluminum and silicon is deposited using two or more source gases, reaction temperatures of which are largely different from each other, such as triisobutyl aluminum gas and silane gas, it is difficult for conventional thermal CVD to mix silicon into aluminum at a low reaction temperature at which the surface of a deposited thin film is smoothed.
As one of methods of preventing the above problem, a plasma CVD method or a photo CVD method which promotes reaction using energy other than heat such as plasma or light has been receiving a great deal of attention. In these CVD methods, although the substrate temperature can be decreased as low as that of vacuum evaporation or sputtering, film quality is degraded or a film is damaged, so that many characteristics of the CVD method are sacrificed. In addition, it is difficult to perform selective growth when another energy such as plasma is used.
The second drawback of CVD is that surface roughness tends to be enlarged. This problem is posed when a crystalline thin film such as a metal film is deposited by thermal CVD. The degree of roughness is normally increased when a surface free energy of a thin film material is high and a substrate temperature is also high. This will be explained as follows in accordance with the nucleus growth theory using a surface free energy model by Volmer et al. Note that a reference is "J.P. Hirth and G.M. Pound: Condensation and Evaporation (Macmillan, New York, 1963)".
Atoms reaching the substrate repeat collision and reevaporation and become an aggregate called a cluster in which atoms of a predetermined number or more are bonded with each other. A total free energy G of one cluster is a sum of a change in a free energy during condensation from a gas and a surface free energy of a formed cluster, and is represented as follows: EQU G =(.sigma..sub.0 .multidot.4.pi.r.sup.2 +g.sub.v .multidot.4.pi.r.sup.3 /3).multidot.f(.theta.) (1)
Where:
r: a radius of curvature PA1 .theta.: a contact angle of a cluster with respect to a substrate PA1 f(.theta.)=(2-3 cos .theta.+cos .sup.3 .theta.)/4: a volume factor of a cluster PA1 .sigma..sub.0 : a surface free energy between a gaseous phase and a cluster per unit volume area PA1 g.sub.v : a change in free energy per unit volume when a phase changes from gaseous phase to liquid phase, which is normally negative. Assuming that a equilibrium vapor pressure is p.sub.e, an actual vapor pressure is p, a molecular volume of the gas is .OMEGA., the Boltzmann constant is k, and the absolute temperature is T, it is represented by g.sub.v =-(kT/.OMEGA.)1n(p/p.sub.e) where p/p.sub.e is the degree of supersaturation.
When r which gives the maximum value of G is r* and dG/dr =0, the following equations (2) and (3): EQU r*=-2.sigma..sub.0 /g.sub.v ( 2) EQU G*=(16.pi..sigma..sub.0.sup.3 /3g.sub.v.sup.3).multidot.f(.theta.)(3)
Therefore, if the radius of a cluster is larger than r*, G is decreased as the cluster grows, so that cluster growth continues. On the contrary, if the radius of a cluster is smaller than r*, average growth does not occur. This value of r* is called a critical radius, a cluster of r=r* is called a critical nucleus, and a cluster larger than a critical nucleus is called a stable nucleus. G* is considered to be an activation energy required to generate the stable nuclei. In addition, assuming that the number of adsorption sites per unit area is n.sub.0, a number density n* of critical nuclei formed on a substrate is given by the Boltzmann equation as follows: EQU n*=n.sub.0 exp(-G*/kT) (4)
As is apparent from the equations (2), (3), and (4), the larger the surface energy .sigma..sub.0 is, or the smaller the volume energy change .vertline.g.sub.v .vertline. during cluster generation is, the larger the critical radius r* becomes, and the smaller the number density n* of the critical nuclei becomes. The number density n of the critical nuclei is also affected by the contact angle .theta. in addition to the surface energy of the thin film material or the degree of supersaturation. When the surface free energy of the substrate material is small and that cf the thin film material is large, .theta., i.e., f(.theta.) is increased, and the number density n* of the critical nuclei is decreased. Thus, the critical nuclei stochastically generated on the substrate continuously grow as stable nuclei, and then these nuclei coalesce with each other to form a thin film. Therefore, as for surface roughness of the thin film thus formed, when density of the stable nuclei initially generated on the substrate is low, the radius at which nuclei coalesce with each other is large, resulting in large surface roughness of the thin film.
As described above, when a thin film material with a large surface free energy is deposited on a substrate with a small surface free energy in a state wherein the degree of supersaturation is small, large nuclei are sparsely generated and surface roughness of the thin film is enlarged. The surface free energy is generally small in an insulator such as an oxide and is large in a metal (e.g., aluminum) or silicon. Therefore, when a thin metal film is formed on an insulating substrate, this tendency most significantly appears. In normal vacuum evaporation or sputtering, since the degree of supersaturation is as extremely large as to 10.sup.10 to 10.sup.20, the critical radius is as small as several .ANG. or less, and the density of the critical nuclei is as high as substantially the number of adsorption sites. Surface roughness of the thin film deposited by these deposition methods is sufficiently small, thereby posing no problem. However, in CVD, it is said that the degree of supersaturation is not so large (reference: W. A. P. Claassen and J. Bloem: J. Electrochem. Soc.: Solid-St. Sci. & Tech. 127 (1980) 194.), and the number density of the critical nuclei is so decreased as to affect surface roughness of deposited film.
Practically, in CVD of a thin metal film, surface roughness poses a serious problem in many cases. For example, when refractory metal films such as molybdenum or tungsten film are deposited by vacuum evaporation or sputtering, a thin film having a small grain size of about several hundreds .ANG. and a smooth surface can be obtained. On the contrary, when CVD is used, large projections are formed on the surface by stone wall-like grains with a diameter of 1,000 .ANG.. The reason for this is assumed as follows. That is, in vacuum evaporation or sputtering, substrate temperatures during deposition are usually as low as 300 to 400.degree. C. While, in CVD method, a substrate temperature higher than 500 to 600.degree. C. which is a decomposition temperature of a source gas is required. This high substrate temperature promotes crystal growth during deposition and deposited films tend to have large grain size. In addition, the low number density of critical nuclei caused by small supersaturation in CVD is another main factor.
Surface roughness poses a serious problem especially in a thin aluminum film formed by CVD. That is, although the substrate temperature of 250 to 300.degree. C. is not so much higher than that by vacuum evaporation, projections are formed by about 10% that a film thickness, so that this method is not put into practice. It is also assumed that a cause of surface roughness is based on a nucleus growth process. A reference for this is, e.g., "R. A. Revy, M. L. Green and P. K. Gallagher: J. Electrochem. Soc.: Solid-St. Sci. & Tech. 131 (1984) 2175". FIG. 11 shows a photograph of a typical metal texture of aluminum deposition by CVD. In FIG. 11, surface roughness caused by crystal grains is present, and gaps are partially formed between the grains.
When large roughness is formed on the thin film surface, it is difficult to form micropatterns with high accuracy by photolithography, and variations occur in the thin film characteristics such as an electrical resistance. In addition, when another thin film is deposited on this thin film to form a multilayer structure, partial variations occur in film thickness at projections, so that an ideal multilayer structure cannot be obtained. As a result, a serious problem is posed when it is used as a material of a semiconductor IC and the like. Micropatterning and high accuracy are required for a semiconductor IC along with its high speed operation and high packing density. In a submicron device or a nanometer device, such a reduction in thin film roughness is a most serious subject.
Problems of conventional CVD can be summarized as follows.
(1) It is very difficult to form a smooth thin film, especially a metal thin film with small surface roughness.
(2) It is difficult to selectively grow a thin film because a high substrate temperature is required during film deposition. In addition, degradation or malfunction of a semiconductor device caused by changes in physical and chemical states of a substrate and a deposited film during film deposition cannot be eliminated.
(3) It is difficult to deposit metal alloy films using two or more source gases whose reaction temperatures are different from each other. Further, it is impossible to deposit aluminum-silicon alloy films with smooth surface, because source gases of silicon such as silane are not decomposed at such low temperatures as surface topography of deposited aluminum film is smooth.